同じ年齢の学生の頻度をカウントするMySQLクエリ？

これには、COUNT（*）をGROUPBYと一緒に使用できます。まずテーブルを作成しましょう-

mysql> create table DemoTable
   (
   StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   StudentAge int
   );
Query OK, 0 rows affected (0.59 sec)

挿入コマンド-

mysql> insert into DemoTable(StudentAge) values(16);
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable(StudentAge) values(17);
Query OK, 1 row affected (0.20 sec)
mysql> insert into DemoTable(StudentAge) values(18);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(17);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(17);
Query OK, 1 row affected (0.23 sec)
mysql> insert into DemoTable(StudentAge) values(17);
Query OK, 1 row affected (0.08 sec)
mysql> insert into DemoTable(StudentAge) values(18);
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(StudentAge) values(18);
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable(StudentAge) values(19);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable(StudentAge) values(19);
Query OK, 1 row affected (0.07 sec)
mysql> insert into DemoTable(StudentAge) values(16);
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(16);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(StudentAge) values(15);
Query OK, 1 row affected (0.16 sec)

selectステートメントを使用してテーブルのすべてのレコードを表示する-

mysql> select * from DemoTable;

これにより、次の出力が生成されます-

+-----------+------------+
| StudentId | StudentAge |
+-----------+------------+
| 1         | 16         |
| 2         | 17         |
| 3         | 18         |
| 4         | 17         |
| 5         | 17         |
| 6         | 17         |
| 7         | 18         |
| 8         | 18         |
| 9         | 19         |
| 10        | 19         |
| 11        | 16         |
| 12        | 16         |
| 13        | 15         |
+-----------+------------+
13 rows in set (0.00 sec)

以下は、頻度をカウントするためのクエリです-

mysql> select StudentAge,count(*) AS `AgeFrequency` from DemoTable group by StudentAge;

これにより、次の出力が生成されます-

+------------+--------------+
| StudentAge | AgeFrequency |
+------------+--------------+
| 16         | 3            |
| 17         | 4            |
| 18         | 3            |
| 19         | 2            |
| 15         | 1            |
+------------+--------------+
5 rows in set (0.05 sec)